When a font is located in the content folder, and you keep the content folder along with your game, then it shouldn't be much of a problem.
Spritefonts are treated similair like images, They both remain in the contents folder.
I've used a downloadable spritefont before and I could use it on different computers without any problems. I think it's a bit depending on how you're releasing your game. (I've used the release folder after building the game in the release configuration)
I think the best way is to try it out yourself, by putting the game and the content folder on someone else's PC. :)